Hey Grif,
So far I've only used 'AA' batteries while on the road. Garmin 'claims' 20+ hours on a set and they have been pretty accurate on that account (the backlight drains power, but don't need it during daylight). I'll try Lithium ones this summer as they claim to last longer. I'll be delighted if I can stretch 25 hours.
As for the 12v cable, I suspect I'd have to secure the cable with duct tape or something similar as well. I'll give it a go this season. If the batteries are in the unit, it should be fine until the next stop (when you apply the tape!).
Cheers,
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